Abstract. The 3H-3He dating method is applied in a buried-valley aquifer near Dayton, Ohio. The study area is large, not all sampling locations lie along well-defined flow paths, and existing wells with variable screen lengths and diameters are used. Reliable use of the method at this site requires addressing several complications: (1) The flow system is disturbed because of high pumping rates and induced infiltration; (2) tritium contamination is present in several areas of the aquifer; and (3) radiogenic helium concentrations are elevated in a significant number of the wells. The 3H-3He ages are examined for self-consistency by comparing the reconstructed tritium evolution to the annual weighted tritium measured in precipitation; deviations result from dispersion, tritium contamination, and mixing. The 3H-3He ages are next examined for consistency with chlorofluorocarbon ages; the agreement is poor because of degradation of CFCs.Finally, the 3H-3He ages are examined for consistency with the current understanding of local hydrologic processes; the ages are generally supported by hydrogeologic data and the results of groundwater flow modeling coupled with particle-tracking analyses.
IntroductionEstimates of aquifer properties that govern chemical transport have been made with increasing regularity using transient tracers, such as tritium (3H) and chlorofluorocarbons (CFCs), which have known time-dependent concentrations in precipitation and in the soil air, respectively, and thus in water recharged to aquifers. In groundwater flow systems where water residence times are less than 50 years, transient tracers can be used to estimate groundwater ages and velocities, aquifer porosity, and recharge rates, which in turn can be used in calibrating flow and transport models.Investigations of groundwater systems using tritium have commonly used the 1960s bomb peak of tritium as a time This contribution evaluates the usefulness of the 3H-3He method for dating groundwater in a complex buried-valley aquifer in southwest Ohio. Unlike many of the previous studies using this method, the study area is large, not all sampling locations lie along well-defined flow paths, and existing wells with variable diameters and screen lengths (up to 7 m) are used. In addition, successful application of the 3H-3He technique at this site is dependent on overcoming complications that in this combination, have not been important factors in previous 3H-3He studies. These complications include sampling flow systems altered by high rates of pumping, which may cause fractionation of the gases dissolved in the water; sampling groundwater affected by seepage of pollutants from sources associated with industrial, commercial, and military facilities, including a U.S. Department of Energy (DOE) research facility, near which tritium concentrations greatly exceed typical atmospheric precipitation levels; and sampling waters that have high concentrations of radiogenic helium. The radiogenic helium can cause analytical difficulties and further complicates t...